Many geothermal sources currently under investigation or exploitation produce large quantities of hot brine at moderate pressures, typically about 150 psia. Some sources, however, produce fluid mixtures of steam and brine at much higher pressures, for example, 800 psia. In the latter case, the brine is usually very corrosive giving rise to problems in using and disposing of the brine. Recently, geothermal wells in Hawaii have been drilled producing high pressure fluid that is about 80% steam and 20% brine. The steam is usually only saturated, and there is some question as to whether these wells will maintain the 800 psia pressure in the face of continuous use over the years.
To take this uncertainty to account, it has been conventional to install pressure reducing valves into the flow from the well so that a low pressure steam system can be used in the expectation that the high pressure eventually will fall. However, this is a conservative design, and it is costly because over the life of the plant, because a considerable amount of potential power will have been lost.
A back pressure steam turbine driving a generator would seem to be an alternative approach in that the high pressure steam from the well could be converted to lower pressure steam by the turbine and applied in parallel to a plurality of modules that can operate on low pressure steam. Each module may utilize a low pressure steam turbo-generator, and a condenser that acts as a vaporizer for an organic vapor turbo-generator. When the geothermal fluid produces only high pressure, saturated steam, expansion of the steam in a turbine takes place in the wet region of the temperature-entropy diagram, producing exhaust steam that contains water droplets and is thus not suitable for application to the inlet stages of low pressure steam turbines in the various modules.
It is therefore an object of the present invention to provide a new and improved geothermal power plant capable of operating on high pressure geothermal fluid without the attendant disadvantages of the prior art described above.